Sexually transmitted infections (STIs) spread through body fluids, skin-to-skin contact, shared blood, and from mother to baby during pregnancy or birth. Sexual contact is by far the most common route, but it’s not the only one, and the specific risks vary depending on the type of sex and the infection involved.
Body Fluids vs. Skin Contact
STIs fall into two broad categories based on how they spread. Some require an exchange of body fluids like semen, vaginal fluids, or blood. HIV, chlamydia, gonorrhea, and hepatitis B all spread this way. Others pass through direct skin-to-skin contact with an infected area, even when no fluids are exchanged. Herpes, HPV, syphilis, and molluscum contagiosum belong to this group.
This distinction matters because it affects how well protection works. Condoms are highly effective at blocking fluid-based infections like HIV, but they offer less protection against skin-to-skin infections like herpes and syphilis. If the infected skin falls outside the area a condom covers, transmission can still happen.
Vaginal and Anal Intercourse
Vaginal and anal sex carry the highest risk for most STIs because they involve prolonged contact with mucous membranes, the thin, absorbent tissue that lines the genitals and rectum. These surfaces are more vulnerable to infection than regular skin.
Anal sex is riskier than vaginal sex for HIV specifically. The receptive partner during anal sex faces roughly a 1 in 71 chance of acquiring HIV per exposure from an infected partner, regardless of whether that partner is male or female. For receptive vaginal sex, the estimated risk is about 1 in 1,250 per exposure. The insertive partner faces lower but still real risk in both cases: approximately 1 in 909 for anal sex and 1 in 2,500 for vaginal sex.
Those numbers change significantly when other STIs are present. Having an existing infection like chlamydia or bacterial vaginosis can increase the risk of acquiring HIV by up to eight times, potentially pushing the per-exposure risk for receptive vaginal sex closer to 1 in 100. This is one reason treating any STI promptly matters even if symptoms feel mild.
Oral Sex Is Not Risk-Free
Many people underestimate oral sex as a transmission route, but several infections spread readily this way. Chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV can all be transmitted through oral sex. You can get an infection in your throat from performing oral sex on an infected partner, and you can get an infection on your genitals from receiving oral sex from someone with a mouth or throat infection.
Throat infections deserve particular attention because they often cause no symptoms and can go undetected for months. A gonorrhea infection in the throat can spread through the body, and certain strains of HPV that infect the mouth and throat are linked to oral and neck cancers. Oral-anal contact (rimming) adds another layer of risk, potentially transmitting hepatitis A, hepatitis B, and intestinal infections caused by parasites and bacteria.
You can carry an STI in more than one location at once. It’s entirely possible to have gonorrhea in your throat and your genitals simultaneously, picked up from different encounters or different types of contact in the same encounter.
Non-Sexual Transmission
A few STIs spread through routes that have nothing to do with sex. HIV and hepatitis B and C can pass through shared needles or other equipment used to inject drugs. Any activity that exposes you to infected blood carries some degree of risk, though occupational transmission (like a healthcare worker getting a needlestick) is extremely rare. Only 58 confirmed cases of occupational HIV transmission have ever been reported in the United States.
Pregnant women can pass infections to their babies during pregnancy, labor, or delivery. The consequences can be severe. Without preventive treatment, up to 90% of infants born to mothers with hepatitis B become infected. About 6 in 100 babies born to mothers with hepatitis C contract the virus. Syphilis poses an especially grave threat: roughly 40% of babies born to women with untreated syphilis are stillborn or die shortly after birth. Congenital syphilis has increased nearly 700% over the past decade in the U.S., with close to 4,000 reported cases in 2024 alone.
What About Toilet Seats and Surfaces?
The bacteria, viruses, and parasites that cause STIs generally cannot survive on hard surfaces like toilet seats. Bacterial STIs die quickly outside the human body. A few viral infections like herpes and hepatitis B can survive briefly on surfaces, but the conditions required for transmission from a surface to a person essentially don’t occur in real-world settings. You are not going to catch an STI from a toilet seat, a doorknob, or a swimming pool.
Most Infections Cause No Symptoms
One of the most important things to understand about STI transmission is that the majority of infections produce no visible symptoms. The World Health Organization estimates that more than one million curable STIs are acquired every day worldwide, and most of those are asymptomatic. This means you can’t rely on looking or feeling healthy as evidence that you or a partner are infection-free.
Chlamydia and gonorrhea frequently cause no symptoms at all, particularly in vaginal infections. Syphilis often produces symptoms too mild to notice, with its characteristic sore sometimes appearing in hidden locations like the cervix or rectum. Herpes can shed virus from the skin even between visible outbreaks. This silent transmission is the primary reason STIs continue to spread so widely.
Testing Timelines After Exposure
If you’ve had a potential exposure, testing too early can produce a false negative. Each infection has a window period, the time between exposure and when a test can reliably detect it.
- Chlamydia and gonorrhea: Tests typically turn positive within one week, though waiting two weeks catches nearly all infections.
- Syphilis: Blood tests pick up most infections by one month, but three months catches almost all cases. The incubation period ranges from 10 to 90 days, averaging about three weeks.
If you test negative shortly after exposure, a follow-up test after the full window period gives you a more reliable answer. Chlamydia and gonorrhea symptoms, when they do appear, typically show up within one to two weeks. But waiting for symptoms is not a substitute for testing, since many infections never produce any.
The Current Landscape
More than 2.2 million cases of chlamydia, gonorrhea, and syphilis were reported in the United States in 2024. While that total declined 9% from the previous year (the third consecutive annual drop), it remains 13% higher than a decade ago. Chlamydia and gonorrhea cases have been falling, and primary and secondary syphilis declined 22% from 2023. The major exception is congenital syphilis, which has risen for 12 straight years, reflecting gaps in prenatal screening and treatment.
These numbers only capture diagnosed and reported cases. Given how many infections are asymptomatic, the true burden is considerably higher. Regular screening remains the most reliable way to catch infections early, prevent complications, and stop the chain of transmission to others.